Certain embodiments of the present invention generally relate to a second audio program (SAP) channel, and in particular, relate to demodulation of a frequency modulation (FM) SAP channel.
Many television programs are enjoyed by audiences speaking a variety of different languages. Therefore, it is desirable to provide audio content in multiple languages along with video content. Additional audio content is typically easier to encode and transmit than video content, so allowing transmission of multiple audio streams with a single video stream may reduce bandwidth and system complexity requirements.
Additionally, information may be provided to supplement an audio or audiovisual program. For example, advertisements or promotions related to a television program may be attractive to viewers. Also, customers may enjoy listening to local radio stations through their television sets or may enjoy receiving current weather or traffic reports along with regular television programming.
SAP channels may be used in audio communications, such as television or radio, for example, to provide a separate audio source in addition to left and right stereo signals or a monoaural audio signal. An SAP channel may be used to duplicate audio content contained on other audio channels or may carry additional information, such as audio content in a second language, descriptive information regarding the audio content, advertising material, local radio stations, weather or traffic reports, frequency-shift keying information, or other additional material unrelated to the main audio content.
During the 1980s, the Federal Communications Commission (FCC) adopted the format established by the Broadcast Television Standards Committee (BTSC) as a standard for multichannel television sound (MTS). Typically, the BTSC format is used with a composite TV signal that includes a video signal, as well as the BTSC format for sound reproduction.
The BTSC format is similar to FM stereo, but has the ability to carry two additional audio channels. Left plus right channel mono information is transmitted in a way similar to stereo FM in order to ensure compatibility with monaural television receivers. A 15.734 kHz pilot signal is used, instead of the FM stereo 19 kHz pilot signal, which allows the pilot signal to be phase-locked to the horizontal line frequency. A double sideband-suppressed carrier at twice the frequency of the pilot transmits the left minus right stereo information. The stereo information may be DBX encoded to aid in noise reduction. An SAP channel is located at 5 times the pilot frequency. The SAP channel may be used for second language or independent source program material. A professional audio channel may be added at 6.5 times the plot frequency in order to accommodate additional voice or data.
Stereo tuners and demodulator units capable of decoding the BTSC format have been on the market for some time. The front end of the units typically includes analog components or integrated circuit chips that cause variation in the amplitude of the composite signal, including the BTSC portion of the signal. This variation in amplitude reduces stereo separation of the right and left channel information carried in the composite signal. Additionally, current stereo tuners and demodulator units attempt to separate an SAP channel using an unnecessarily complicated process with significant hardware cost.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with embodiments of the present invention as set forth in the remainder of the present application with reference to the drawings.
It is desirable to perform BTSC decoding in the digital domain on a block of an ASIC chip such that the implementation is optimized for reduced complexity and cost. By reducing the complexity, fewer clock cycles are required for processing, and power consumption is also reduced.
Thus, there is a need for a system that isolates and demodulates an SAP channel without unnecessary noise or variation. Also, there is a need for a simplified and efficient method of demodulating an SAP channel. There is a further need for a system that demodulates an SAP channel of an audio communication without excess hardware. There is a need for a system that utilizes an efficient approximation of the digital FM demodulation equation.